ELECTRONIC LETTER A comparative study between infertile males and patients with Turner syndrome to determine the influence of sex chromosome mosaicism and the breakpoints of structurally abnormal Y chromosomes on phenotypic sex

The Y chromosome is important for male development as it contains the sex determining gene SRY and many spermatogenesis genes. Structural abnormalities of the Y chromosome include rings, deletions, inversions, and dicentrics. 4 These types of abnormalities are common in infertile males (1.5%), especially those with azoospermia. 6 However, such rearrangements are unstable and an additional 45,X cell line is frequently present. The 45,X cell line has been shown to influence phenotypic sex so that these chromosome constitutions may also be found in patients with ambiguous genitalia and in female patients with gonadal dysgenesis and Turner syndrome. 7 In fact, from cytogenetic studies about 4-6.2% of female Turner patients show Y chromosome mosaicism irrespective of the presence of SRY. 11 12 Mosaicism varies widely between tissues and accurate interpretation depends on the number of cells examined and the number and types of tissues studied. 14 It has been reported that phenotypic sex is strongly influenced by the percentage and distribution of Y chromosome containing cells in the gonads. 16 However, studies on gonadal tissue are hindered by the fact that it is rarely available for analysis and alternative, more easily accessible tissue is usually studied. It has also been suggested that the structure of the Y chromosome may indirectly affect phenotypic sex. The repetitive sequences at the euchromatin/heterochromatin boundary of the Y chromosome long arm are thought to have an important stabilising role and loss of this region loses this effect, resulting in mosaicism with a 45,X cell line. In dicentrics, which are the most common abnormality of the Y chromosome, it has been suggested that the position of the q arm breakpoint in dicentric Yp chromosomes can influence Y chromosome stability. The more proximal the long arm breakpoint, the greater the instability of the dicentric Yp chromosome which results in a higher percentage of 45,X cells and a sex phenotype which is more likely to be female. In this study, we have used a combination of G banded cytogenetic analysis and FISH to compare levels of sex chromosome mosaicism in different tissues from infertile males and female Turner patients, with a similar mosaic karyotype. FISH and PCR were also used to characterise deletion breakpoints of abnormal Y chromosomes to see whether they were related to the stability of the Y chromosome and therefore indirectly influenced phenotypic sex through levels of 45,X mosaicism.